PCYT1B (choline-phosphate cytidylyltransferase B, CCTβ) is the beta isoform of CTP:phosphocholine cytidylyltransferase (CCT), the enzyme that catalyzes the rate-limiting step of the CDP-choline (Kennedy) pathway of phosphatidylcholine biosynthesis. It converts phosphocholine and CTP into CDP-choline and diphosphate (EC 2.7.7.15). Like the CCTα paralog (PCYT1A), it is an amphitropic, membrane-activated enzyme whose activity depends on binding to lipid regulators; a catalytic cytidylyltransferase domain is followed by amphipathic helical repeats that sense and dock onto membranes, interconverting a soluble low-activity form and a membrane-bound active form. Unlike nuclear CCTα, CCTβ lacks the N-terminal nuclear localization signal and is localized to the cytoplasm and endoplasmic reticulum, where the active homodimer condenses phosphocholine and CTP. CCTβ has a more tissue-restricted expression pattern than the ubiquitous CCTα, with high transcript levels in gonad (testis, ovary), placenta, and brain, consistent with a role in supplying phosphatidylcholine in neural and reproductive tissues. Multiple alternatively spliced isoforms exist (β1 and β2 differ at the C-terminus and in their degree of phosphorylation).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004105 choline-phosphate cytidylyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Core enzymatic activity of PCYT1B, inferred phylogenetically across the CCT family. Directly supported experimentally for this gene (see the IDA/TAS annotations to the same term) and by the UniProt EC 2.7.7.15 assignment. Supporting Evidence: file:human/PCYT1B/PCYT1B-uniprot.txt EC=2.7.7.15 {ECO:0000269|PubMed:10480912, ECO:0000269|PubMed:9593753} |
| GO:0031210 phosphatidylcholine binding | IBA GO_REF:0000033 | ACCEPT | Summary: Reflects the membrane-lipid-sensing property of CCT enzymes. CCTβ, like CCTα, has amphipathic helical repeats and requires lipid regulators for maximal activity, consistent with membrane phospholipid (PC) binding. Reasonable family-level inference. Supporting Evidence: PMID:9593753 Like CCTalpha, CCTbeta required the presence of lipid regulators for maximum catalytic activity. |
| GO:0003824 catalytic activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Correct but uninformative root-level catalytic term. The specific activity (choline-phosphate cytidylyltransferase, GO:0004105) is directly annotated with experimental evidence, so this generic term is an over-annotation. |
| GO:0004105 choline-phosphate cytidylyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic assertion of the core enzymatic activity (via RHEA:18997 / EC:2.7.7.15). Redundant with the experimental IDA/TAS annotations to the same term but not incorrect. Supporting Evidence: file:human/PCYT1B/PCYT1B-uniprot.txt EC=2.7.7.15 {ECO:0000269|PubMed:10480912, ECO:0000269|PubMed:9593753} |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Supported by experimental localization; CCTβ (unlike nuclear CCTα) is cytoplasmic. Consistent with UniProt subcellular location for isoform 1. Supporting Evidence: PMID:9593753 and CCTbeta protein was localized to the cytoplasm as detected by indirect immunofluorescent microscopy. |
| GO:0005783 endoplasmic reticulum | IEA GO_REF:0000044 | ACCEPT | Summary: ER localization is well established experimentally for both CCTβ splice variants; the active PCYT1 homodimer acts at the ER membrane. Supporting Evidence: PMID:10480912 both CCTbeta1 and CCTbeta2 localized to the endoplasmic reticulum of cells |
| GO:0006657 CDP-choline pathway | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic assertion of the specific pathway in which CCTβ acts. Directly supported by the experimental IDA annotation and by the UniProt FUNCTION statement. Supporting Evidence: file:human/PCYT1B/PCYT1B-uniprot.txt Catalyzes the key rate-limiting step in the CDP-choline pathway for phosphatidylcholine biosynthesis. |
| GO:0046470 phosphatidylcholine metabolic process | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: True but general; PCYT1B specifically participates in phosphatidylcholine biosynthesis (GO:0006656) and the CDP-choline pathway (GO:0006657), which are more informative child terms. Retained as non-core context. |
| GO:0006656 phosphatidylcholine biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: Core biological process. Electronic assertion redundant with the experimental IDA/TAS annotations to the same term; well supported. Supporting Evidence: file:human/PCYT1B/PCYT1B-uniprot.txt the CDP-choline pathway for phosphatidylcholine biosynthesis. |
| GO:0004105 choline-phosphate cytidylyltransferase activity | IDA PMID:10480912 Distribution of CTP:phosphocholine cytidylyltransferase (CCT... | ACCEPT | Summary: Experimentally demonstrated enzymatic activity for CCTβ2; the protein has in vitro activity and complements the cytidylyltransferase defect in CHO58 cells. Core molecular function. Supporting Evidence: PMID:10480912 CCTbeta2 protein has enzymatic activity in vitro and was able to complement the temperature-sensitive cytidylyltransferase defect in CHO58 cells |
| GO:0005783 endoplasmic reticulum | IDA PMID:10480912 Distribution of CTP:phosphocholine cytidylyltransferase (CCT... | ACCEPT | Summary: Direct immunolocalization of both CCTβ splice variants to the ER, contrasting with nuclear CCTα. Core location. Supporting Evidence: PMID:10480912 both CCTbeta1 and CCTbeta2 localized to the endoplasmic reticulum of cells, in contrast to CCTalpha which resided in the nucleus |
| GO:0006656 phosphatidylcholine biosynthetic process | IDA PMID:10480912 Distribution of CTP:phosphocholine cytidylyltransferase (CCT... | ACCEPT | Summary: Core biological process, experimentally supported; CCTβ2 complements the cytidylyltransferase defect and, in the companion paper, overexpression enhances PC synthesis. Supporting Evidence: PMID:10480912 CCTbeta2 protein has enzymatic activity in vitro and was able to complement the temperature-sensitive cytidylyltransferase defect in CHO58 cells |
| GO:0006657 CDP-choline pathway | IDA PMID:10480912 Distribution of CTP:phosphocholine cytidylyltransferase (CCT... | ACCEPT | Summary: The specific Kennedy pathway branch in which CCTβ acts; supported by functional complementation of the cytidylyltransferase defect and by accumulation of CDP-choline on CCTβ overexpression (companion paper). Core process. Supporting Evidence: PMID:10480912 CCTbeta2 protein has enzymatic activity in vitro and was able to complement the temperature-sensitive cytidylyltransferase defect in CHO58 cells |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-1483081 | ACCEPT | Summary: The active membrane-bound PCYT1 homodimer condenses phosphocholine and CTP at the ER membrane. More specific than the ER organelle term and consistent with the amphitropic, membrane-activated mechanism. Supporting Evidence: Reactome:R-HSA-1483081 At the endoplasmic reticulum (ER) membrane, active membrane-bound choline-phosphate cytidylyltransferase A (PCYT1A) or B (PCYT1B) homodimer condenses phosphocholine (PCho) and cytidine triphosphate (CTP) to produce CDP-choline (CDP-Cho) |
| GO:0004105 choline-phosphate cytidylyltransferase activity | TAS PMID:9593753 Cloning and characterization of a second human CTP:phosphoch... | ACCEPT | Summary: Cloning and characterization of the second human CCT (CCTβ) established this as a distinct cytidylyltransferase; overexpression drives CCT activity, CDP-choline accumulation, and PC synthesis. Core molecular function. Supporting Evidence: PMID:9593753 Transfection of COS-7 cells with a CCTbeta expression construct led to the overexpression of CCT activity, the accumulation of cellular CDP-choline, and enhanced radiolabeling of phosphatidylcholine. |
| GO:0005737 cytoplasm | TAS PMID:9593753 Cloning and characterization of a second human CTP:phosphoch... | ACCEPT | Summary: CCTβ, lacking the CCTα nuclear localization signal, was localized to the cytoplasm by immunofluorescence. Core location. Supporting Evidence: PMID:9593753 and CCTbeta protein was localized to the cytoplasm as detected by indirect immunofluorescent microscopy. |
| GO:0006656 phosphatidylcholine biosynthetic process | TAS PMID:9593753 Cloning and characterization of a second human CTP:phosphoch... | ACCEPT | Summary: CCTβ overexpression enhances phosphatidylcholine production, establishing its role in PC biosynthesis. Core biological process. Supporting Evidence: PMID:9593753 another mechanism by which cells regulate phosphatidylcholine production. |
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